In the applicant's U.S. Pat. No. 4,627,951 there is disclosed a process for making composite products from sugar containing lignocellulosic material, specifically of annual plants of a non-woody nature, such as sugar cane bagasse, and stalks of corn, sorghum, and sunflower, et cetera. The natural sugars and other water soluble materials within the lignocellulosic material are chemically transformed, in situ, by the application of heat and pressure into an insoluble and infusible polymeric substance, acting as both a bonding and a bulking agent, to strengthen the reconstituted composite products with strong mechanical strength and superb dimensional stability.
However, this patented process is limited to lignocellulosic material containing natural free sugars and other water soluble materials and is not applicable to lignocellulosic materials, such as wood, cereal straws, rice husks, et cetera. The conventional process for making composite panel products from lignocellulosic materials relies exclusively on synthetic thermosetting resin binders for bonding. Since synthetic resins, such as phenol- and urea-formaldehyde, are expensive, they normally constitute a large portion of the production cost for the conventional panel products such as particleboard, waferboard, and medium density fiberboard. This holds specifically true in the case of agricultural residues. Because of their physical nature of agricultural residues, a relatively high content of resin binder is required for manufacturing, thus resulting in expensive panel products. The prohibitive cost of synthetic resin binders is the major reason why agricultural residues are not widely utilized today in the manufacturing of panel products, in spite of the abundance and availability of the raw materials.
In recent years a number of manufacturing processes have been developed to utilize agricultural residues around the world. However, none of the processes developed thus far have found commercial acceptance. Thus E.C. Lathrop et al. in "Hardboard from agricultural residues", Modern Plastics, p 126 (April 1951) reported the use of a combination of powdered thermosetting phenolic resin, pine gum and ground rice husks to make composite panel products. Lathrop et al found that the boards that contained as much as 15% phenolic resin were too brittle to be nailed. Moreover, the boards had a density of 1.12 (69.9 lbs. per cubic foot, pcf). This combination of 15% powdered resin and high density made the product too expensive to compete with existing products, and the brittleness of the board placed a severe limitation on its use.
The use of a specially formulated phenolic resin for bonding rice husks has been reported by R. C. Vasisshth in the U.S. Pat. No. 3,850,667, dated Nov. 26, 1974. According to VAsisshth, rice husk boards can be made with 8 to 10 percent of a water immiscible, caustic free, thermosetting phenol-formaldehyde resin and pre-treated rice husks. In this process, a pretreatment is essential in order to break up the rice husk pods into individual leaves, to remove loosely bonded surface material and to screen out fine particles. It is claimed that the inclusion of fine materials generated from the pretreatment would not only increase the resin consumption, but would also introduce some undesirable effects on the properties of the board.
More recently, "New opportunities in manufacturing conventional particleboard using isocyanate binders" reported by G. W. Ball (Proceeding--Washington State University Particleboard Symposium. No. 15, p 266-285, 1981), teaches manufacturing rice husk boards with pretreated rice husks, using 9% of a very expensive polymeric isocyanate resin as the bonding agent. Since isocyanate resin is more expensive than conventional phenolic resin, the production cost of rice husk board is very high. The high manufacturing cost of this rice husk board prevents it from being competitive with the conventional wood-based panel products.
Present day methods of manufacturing panel products from lignocellulosic materials rely exclusively upon synthetic thermosetting resin for bonding. Synthetic resin binders are expensive because they are derived from petro-chemicals. In general, the resin binder cost constitutes a major portion of the production costs for these panel products, thus limiting the type of lignocellulosic material which may be used, particularly those usually selected from agricultural residues. In view of the high resin binder cost and the limitation on the raw material selection, a process that eliminates synthetic resin for manufacturing of composite panel products and which can be used with any lignocellulosic materials would be very attractive economically and technologically.
Since lignin is believed to be the natural binder within lignocellulose and is phenolic in nature, it has been extensively studied and researched as a binder for lignocellulosic composite products. Over the years different methods have been developed for the conversion of wood and agricultural residues into composite products such as panel and moulded products by generating and releasing the natural component of lignin within lignocellulosic materials for use as a resin binder. The most common method of releasing and reactivating lignin is by subjecting the lignocellulosic material to a drastic hydrolysis in the presence of water or acids at an elevated temperature. The hydrolysis removes the hemicellulose portion of lignocellulosic material, hence increasing the ratio of lignin to cellulose over that which is normally present in lignocellulosic materials and therefore improving bonding efficiency.
The U.S. Pat. No. 726,029 by A. Classen uses steam to treat saw dust with acid and cooks it under pressure at a temperature of 105 to 145 degrees Celsius for 30 to 60 minutes to render the hemicellulose water soluble. At the end of the cooking, the reacted mass is washed with water to remove the water solubles before drying and moulding.
Likewise, Sherrard, et al, in U.S. Pat. No. 2,513,316 cooks dry fibrous vegetable material under pressure in a digestor. The resulting material is then thoroughly washed with water to remove the acid and water soluble reaction products. The remaining material is then subjected to heat action and then ground to a powder for moulding.
Again, Schorger and Ferguson, U.S. Pat. Nos. 2,196,277, 2,247,204 and 2,283,820, teach cooking a natural lignocellulosic material with water or with added materials to render a part of the lignocellulosic material water soluble and particularly to dissolve the hemicellulose. The residual products, after extraction of the water solubles and subsequent drying, contain a larger part of the original thermoplastic resinous lignin.
U.S. Pat. No. 2,303,345 by Mason and Boehm describes a process of making tough products from lignocellulosic material. Mason and Boehm use high pressure stream to separate lignin from the lignocellulosic material for bonding, in that hemicellulose is hydrolyzed into water solubles which are removed from the treated lignocellulose before the fibers are made into hardboard. Consequently the removed water solubles are processed separately as a by-product with a trade mark of "Masonoid". In U.S. Pat. No. 2,224,135, issued to R. Boehm, "Masonid", the water solubles by-products from hardboard manufacturing, are used in making aldehydes, alcohols and organic acids. This patent also mentions that the water solubles thus obtained can be further concentrated and used as a water soluble adhesive. The use of "Masonid" as a water soluble adhesive is also taught in U.S. Pat. Nos. 2,643,953 and 2,716,613, issued to W. Schoel in 1949 and 1950 respectively. In these patents, it is stated that while these water solubles have adhesive properties, it has been found that they are not entirely satisfactory for use as an adhesive. One reason given is that these water solubles are undesirably hygroscopic and therefore, the bond formed by them in adhesive application is somewhat unstable. Under high humidity the adhesive bond formed by these water solubles absorbs moisture from the air, thereby weakening the adhesive bond, whereas under low humidity, the adhesive bond formed by these water solubles loses moisture and also weakens. Upon absorbing moisture, the adhesive bond formed by these water solubles tends to liquefy, while moisture loss tends to harden the board so that is approaches a state of brittleness (U.S. Pat. No. 2,643,953, Col. 1, Lines, 25-35).
Boehm and Schoel were not aware that these water solubles are thermosetting and capable of being used as a water-proof adhesive if proper processing application is followed. Instead, according to Boehm and Schoel patents the "Masonoid" may be used only as a water-soluble adhesive. Since they did not recognize the bonding nature of the water soluble materials, their patents did not teach thermosetting in their respective processes and the resulting bond which is not thermoset, is therefore not water-proof and has only limited commercial application.
The lack of appreciation and understanding of the potential of both Boehm and Schoel that the water solubles from hemicellulose hydrolyzation are capable of being thermoset into a water-proof adhesive bond, which is physically and chemically stable and resistant to boiling water, may be attributed to the logical development of the Masonite process in that the natural lignin is used as a binder, not the water soluble materials, which are removed as "Masonoid" in the Masonite process. The removal of water solubles is a key feature of Masonite process for making board products.
A similar Masonite process is taught in U.K. Patent 497,477 issued to W. W. Triggs in 1938, using steam at a temperature in the range from 216 to 285 degree Celsius with a time range from 12 seconds to 30 minutes to treat the lignocellulosic material for moulded products. Again, Triggs relies on the lignin for bonding and specifies the removal of water solubles generated during the steam treatment in order to obtain a high quality product.
All the processes mentioned above use lignin as a binder and remove the water solubles during the processes; therefore, requiring an enormous amount of water for processing. The commercial use of the Masonite process, which consumes a particularly large quantity of water has caused serious water pollution as well as other environmental concerns. This is one of the main reasons for the decline of the use of the Masonite process for making headboard since World War II. Today there are only a few remaining Masonite plants operating around the world. The only Masonite plant built in Canada was closed down in 1985.
Glab describes in 16 U.S. Pat. Nos. (2,706,160 the first and 3,252,815 the latest) the treatment of lignocellulosic material with high temperature steam for long duration in the presence of a chemical reactant capable of splitting at least a portion of lignin, which is used as a thermoplastic binder. In the examples cited in the Glab patents, steam at a temperature in the range of 218-232 degree Celsius to 254-288 degree Celsius is used with a treating time in the range between 20-30 minutes to 4-5 minutes. During this severe steam treatment, not only are the water solubles from hemicellulose hydrolysis inevitably further transformed by polymerization into a high molecular weight product, which is retained and used as a plasticiser in moulding operation, but the process also purposely reduces the alpha cellulose in molecular size to prevent swelling of moulded products (U.S. Pat. No. 2,984,580, Col. 1, Lines 34-38). In contrast to Classen, Sherrard, Schorger, Mason, Boehm and Triggs, all of whom remove water solubles from hemicellulose hydrolysis in their processes, Glab claims that the full utilization of lignocellulosic material is made by retaining the polymerized water solubles from hemicellulose as a plasticizer in the moulding operation, while the catalyzed lignin is used as a binder. Glab also claims that the flow of the mouldable material is improved by his process and a short time is required if a plasticizer is added (U.S. Pat. No. 2,984,578, Col. 4, Lines 20-23). Glab identifies and states that the preferred plasticizers are water, furfural, anile and phenol in a preferred quantity between 2-20%. Glab, like Classen, Sherrard, Schorger, Mason, Boehm and Triggs, was not aware, as the applicant has found, that the water soluble decomposition material from hemicellulose hydrolysis carried out in a mild steam treatment can be used as a thermosetting water-proof adhesive binder. Instead, Glab used very high steam temperature in combination with a long steaming duration and in the presence of a chemical reactant to break down lignin and alpha cellulose. In his process, Glab inevitably and unintentionally over cooked the hemicellulose and destroyed it beyond use as a binder. Glab polymerized, thermoset and converted the water solubles derived from hemicellulose decomposition and hydrolysis to high molecular weight materials which, after being thermoset could be used as plasticizers for the treated lignocellulose when moulded, while lignin is made to flow and function as a binder between the comminuted lignocellulose particles (U.S. Pat. No. 2,984,580, Col. 2, Lines 1-3). The steaming conditions (the combination of temperature and time) used by Glab were much more severe than those employed by Triggs. In comparison, Triggs' treating conditions are much too harsh to result in the present invention. The comparative severity of steam treating conditions employed by Triggs, Glab and the applicant is clearly shown in FIG. 1.
All processes known to the applicant, which use natural lignin as a binder, have one unique characteristic in that the final lignocellulosic moulding product is very dense and heavy. For any structural application, the specific gravity of the moulded products is always in the range of 1.0 to 1.4 (62.4 to 87.4 pcf). This is consistent with the use of lignin as a binder, resulting from the required very high moulding pressure which is necessary to make the lignin flow and bind. Furthermore, the severe treatment by high steam temperature for a long time in the known processes has also caused damage to the structural integrity of cellulose with a much lower degree of polymerization (D.P), particularly in the case of Glab, who specifies the controlled degradation so that the alpha cellulose is reduced in molecular size sufficiently to prevent swelling of the moulded products (U.S. Pat. No. 2,984,580, Col. 1, Lines 35-37). The damaged cellulose fiber, i.e. that with the lowered D.P., being the only structural component the moulded products has to rely on for the high density to compensate for the physical weakness imparted to the product, caused by the severe treatment. Thus high density products produced by these known processes have limited uses.